Among the so-called composite materials, those composite materials which are composed of such brittle materials as ceramics and the like have been developed as structural materials or functional materials, and encompass conventional rather macroscopic materials with particles, fibers, and the like dispersed in the matrices thereof and recent composite mesoscopic materials and nanocomposite materials designed for the composite formation on the crystal level, the recent ones being highlighted. The nanocomposite materials include the intra-crystal nanocomposite type in which nanosize crystals of other materials are introduced either into the interior of a grain or into the grain boundary, and the nano-nanocomposite type in which nanosize crystals of different materials are mixed. Some nanocomposite materials are expected to display hitherto unknown characteristics, and related research papers have been published.
In NEW CERAMICS (1997: No. 2), there is found a description that a raw material is produced in which the ultra-fine particles made of zirconia surround the particles of an alumina raw powder, and the raw material thus produced is sintered to yield a nanocomposite.
In New Ceramics (in Japanese) (1998, Vol. 11, No. 5), there is found a description that a composite powder is produced by depositing Ag particles or Pt particles on the surface of a PZT raw powder in such a way that the surface of ceramic fine particles undergoes a chemical process such as the electroless plating method, and the composite powder thus obtained is sintered to yield a nanocomposite.
Additionally, in New Ceramics (in Japanese) (1998, Vol. 11, No. 5), there is found a description that as the materials for use in preparing nanocomposites, there can be cited Al2O3/Ni, Al2O3/Co, Zr2O/Ni, Zr2O/SiC, BaTiO3/SiC, BaTiO3/Ni, ZnO/NiO, PZT/Ag, and the like, and the sintering of these materials gives nanocomposites.
The nanocomposites disclosed in these articles are all obtained by sintering, which induces the grain growth so that the grain size tends to become coarse and large, and accordingly there occurs such a limitation that the sintering does not lead to oxidation. Additionally, in the case where a composite body composed of a ceramic and a metal is formed, if the sintering temperature of the ceramic and the melting point of the metal are remarkably different from each other, in some case the metal is evaporated at the sintering temperature, and thus there occurs a problem that the control of the composition ratios is difficult, and other like problems. Furthermore, in the case where a metal is plated on the surface of the ceramic powder by the electroless plating and the like, the applicable metal is limited, and there is a fear that the impurity contamination occurs in the wet process.
On the contrary to the above described nanocomposites which are obtained by sintering, in Materials Integration (2000, Vol. 13, No. 4), there is found a description that a variety of Cr/CrOx nanocomposite thin films can be obtained by the reactive low-voltage magnetron sputtering method with a Cr target under the condition that the O2 partial pressure is varied. According to this method, however, it is impossible to conduct the nanosize crystal deposition of mixed fine particles of different types in the form of dispersed particles instead of in the form of laminated layers.
On the other hand, as the recent novel methods of coating film formation, there have been known the gas deposition method (Seiichirou Kashu, Kinzoku (Metals, in Japanese), January, 1989) and the electrostatic fine particle coating method (Ikawa et al., Preprint (in Japanese) for the Science Lecture Meeting, Autumn Convention, Precision Machine Society, Showa 52 (1977)). The fundamental principle of the former method is as follows: the fine particles of metals, ceramics, and the like are converted into aerosols by gas agitation, and accelerated through a fine nozzle so that a part of the kinetic energy is converted into heat when colliding with the substrate, which leads to the sintering found either among the particles or between the substrate and particles. The fundamental principle of the latter method is as follows: the fine particles are charged, accelerated in a gradient of electric field, and the subsequent sintering involves the use of the heat generated in bombardment in a similar manner to that in the former method.
In this connection, as the preceding techniques in which the above descried gas deposition method is applied to mixed fine particles of different types, there have been known the techniques disclosed in Japanese Patent Publication No. 3-14512 (Japanese Patent Laid-Open No. 59-80361), Japanese Patent Laid-Open No. 59-87077, Japanese Patent Publication No. 64-11328 (Japanese Patent Laid-Open No. 61-209032), and Japanese Patent Laid-Open No. 6-116743.
In the contents proposed in the above Japanese Patent Publications, the different types of fine particles are based on such metals (ductile materials) as Ag, Ni, Fe and the like; namely, no specific suggestions are found therein with respect to the formation of the nanocomposites of metals and ceramics (brittle materials) or the composites of organics and inorganics.
Additionally, the techniques described above take as their fundamental principle the film formation composed of mixed fine particles through melting or partially melting the raw material ultra-fine particles, but without using adhesive agents, so that there are involved such auxiliary heating devices as an infrared heating device and the like.
On the other hand, no nanocomposite was cited therein, but the present inventors proposed a method for producing the films of ultra-fine particles, excluding heating with heating measures, in Japanese Patent Laid-Open No. 2000-212766. In the technique disclosed in this Japanese Patent Laid-Open No. 2000-212766, a structure body is formed through promoting the mutual bonding of the ultra-fine particles in such a way that the ultra-fine particles of 10 nm to 5 μm in particle size are irradiated with an ion beam, an atomic beam, a molecular beam, a low-temperature plasma, or the like, in order to activate the ultra-fine particles without melting thereof and blow them onto a substrate at a rate of 3 m/sec to 300 m/sec.
The above described prior arts can be summarized as follows: the prior composites referred to as nanocomposites are obtained by sintering almost without exception, and the sintering is inevitably accompanied by the crystal grain growth, leading to the larger average grain size of the composites as compared to that of the raw material fine particles, and hence inducing the difficulty in obtaining such composites as excellent in strength and denseness; in this connection, a proposal has been made for suppressing the crystal grain growth, but the fact is that there is found some limitation to the types of raw materials to which the proposal is applicable.
Furthermore, even a method of coating film formation with fine particles involving no sintering needs some kind of surface activation procedure, almost no considerations are given to the ceramics, and exactly no reference is made to the nanocomposites composed of brittle materials such as ceramics and ductile materials such as metals.
The present inventors have been engaged in the subsequent check and confirmation investigation on the technique disclosed in Japanese Patent Laid-Open No. 2000-212766. Consequently, the present inventors have been successful in revealing that there is definite difference in behavior between metals (ductile materials) and brittle materials including ceramics and semiconductors.
More specifically, as for the brittle materials, the structure bodies were able to be formed without using the irradiation of the ion beam, atomic beam, molecular beam, low-temperature plasma, or the like, namely, without using any particular activation procedure, although there was still a problem that the structure bodies were unsatisfactory in the peel strength or partially tended to be peeled off or the density is not uniform, when there were implemented just the fine particle size of 10 nm to 5 μm and bombardment velocity of 3 m/sec to 300 m/sec as specified in the conditions described in the above mentioned patent laid-open.
On the basis of the above described considerations, the present inventors reached the following conclusions.
The ceramics take the atomic bonding condition that the free electrons are scarcely found and the covalent bonding or the ionic bonding is predominant. Thus, they are hard but brittle. The semiconductors such as silicon, germanium and the like are also brittle materials without ductility. Accordingly, when mechanical impact is exerted to the brittle materials, for example, the crystal lattice dislocation occurs along such a cleavage plane as the boundary face of the crystallites, or the fracture occurs. Once these phenomena have occurred, there are found such atoms as exposed on the dislocation plane and the fracture plane, although these atoms have been originally located in the interior where they have been bonded to other atoms; namely, a new surface is thus formed. The atomic single layer part on the new surface is forced by the external force to make transition to the exposed and unstable surface state from the originally stable atomic bonding state, giving rise to, in other words, a high surface energy state. This activated surface is bonded to the adjacent surface of the brittle material as well as another adjacent new surface of the brittle material or the adjacent substrate surface, thus being converted to a stable state. Exertion of continuous, external mechanical impact makes this phenomenon to occur continuously, and the accompanying repeated distortion and fracture of the fine particles lead to the joining development, densifying the thereby formed structure body. Thus, the structure bodies of the brittle materials are formed.